The present invention relates to a method of making an internal and/or external thermally-protective coating for a thruster structure, in particular a structure forming part of a solid-propellant thruster. The invention also relates to a method of making a thruster structure, and to the thruster structure obtained thereby.
The structure of a solid propellant thruster essentially comprises a casing, e.g. made of composite material and generally provided with an internal thermally-protective coating that needs to perform three essential functions: providing the composite casing with thermal protection against attack from the hot gas that results from combustion of the propellant; attenuating the mechanical stresses generated by the casing deforming under pressure during combustion of the propellant; and sealing the casing against gas leaks.
Various methods exist for applying thermally-protective coatings to the inside of the casing of a thruster structure. One of them consists in starting by using means that are conventional in the rubber industry (open mills, kneaders, . . . ) to prepare a rubber of viscous consistency in the semi-manufactured non-vulcanized state, and in transforming the rubber into elastomer sheets for cutting out and then draping on a mandrel prior to performing vulcanization in an autoclave. The various thermal protection elements formed in that way are then disassembled from their respective mandrels in order to be assembled on another mandrel (generally a dismountable mandrel made of metal) used for winding the filaments of the composite casing onto the thermal protection prepared in that way. That method leads to long manufacturing cycles which cause that technology to be particularly expensive to implement. It requires a large amount of tooling and also presents a succession of operations that are discontinuous and some of which are manual. The use of a plurality of different mandrels during the various steps in implementation of the method is also time-consuming and lengthens the duration of the manufacturing cycle.
Another type of known method enables implementation costs to be reduced. It consists in covering a mandrel in a layer of elastomer prior to forming the casing of the thruster structure by winding the filament of a composite material. In such a method, the elastomer layer is made by depositing an extruded strip over the entire outside surface of a rotating mandrel. The coating obtained in that way is then vulcanized in an autoclave prior to winding on the filament. Although such a method simplifies the method of making the internal thermally-protective coating, it still requires complex tooling to be used such as an extruder, and therefore still presents implementation costs that are high. In particular, it is necessary to vulcanize the coating in order to give it the desired mechanical and thermal characteristics. The operation of vulcanization in an autoclave takes place under the combined effects of pressure (generally of the order of 1 megapascal (MPa) to 3 MPa), and of temperature (typically of the order of 140° C. to 180° C.). As a result, it is necessary for the mandrel to be mechanically dimensioned relative to the autoclave pressure, which leads to mandrel designs that are much more complex than would be necessary when using a mandrel specific for the operation of winding the filament of the structure.
Furthermore, flexible thermally-protective coatings make use of rubbers (a specific association of ingredients) that are specially formulated to perform the three above-specified main functions, i.e. withstanding ablation in the face of thermal and mechanical attack from the propergol combustion gas, providing thermal insulation for the structure, and attenuating mechanical stresses. In addition, given that optimizing the performance of a solid propellant thruster requires its dead weight to be reduced (i.e. including the weight of its internal thermal protection), the ideal material for forming this internal thermal protection needs to present very good resistance to ablation by the thermal mechanical attack from the combustion gas, associated with low density, and low thermal conductivity. Unfortunately, the formulation techniques for obtaining good resistance to ablation and the formulation techniques for obtaining low density (which is generally associated with low thermal conductivity) are mutually antagonistic, so that when only one material is used for performing the thermal protection function, it is necessary to find a compromise in terms of thermal characteristics and ablation characteristics. Finding such a compromise generally leads to a solution that is not good for thruster performance. To mitigate that drawback, it is possible to envisage thermal protection solutions that include function gradients. Such solutions consist in using a material having good resistance to ablation, generally associated with high density, for those layers of the coating that are directly exposed to the combustion gas, while using a material of low density, generally associated with low thermal conductivity, for the underlying layers that are not exposed throughout the time the thruster is in operation. However such solutions are very rarely applied because they lead to additional manufacturing costs, both in terms of preparing the rubber in a non-vulcanized semi-manufactured state, and in terms of actually fabricating the thermal protection elements.